There are several shortcomings realized during starting and idle or low speed running of two-cycle engines. Among these shortcomings is a significant force required to move the pistons through the compression phase of their cycle during starting. This requires an unduly large pull force on the rope of a rope-start two-cycle engine. It is desirable to reduce the pulling effort required to start two-cycle engines. Also, after a two-cycle engine has been started, it suffers the shortcoming of producing significant engine shake and significant exhaust noise at idle speeds.
It is known to reduce the magnitude of each of the aforementioned shortcomings by providing an idle relief aperture in one or more of the cylinder walls of the engine at a height significantly above the cylinder's respective exhaust port which leads into a passage which communicates with the exhaust runner. The aperture and passage thereby provide a path for high pressure exhaust blow-down gas to flow from a point in the cylinder significantly above the exhaust port window to a point in the exhaust runner leading to atmosphere. While this has been found to be effective in reducing the requisite starting effort and reducing engine shake and exhaust noise level at low speeds, the provision of the idle relief aperture leading to the exhaust runners results in a significant horsepower loss throughout the normal operating range of the engine. There is a need for an engine having an improved idle relief structure which provides reduction of starting effort, reduced engine shake and reduced exhaust noise level for an engine operating at idle or low speed, which reduces the degree of horsepower loss of the engine.